This invention relates to an improved device for measuring diametral changes in a substantially cylindrical member. It relates particularly to a device for measuring slight diametral changes in a substantially cylindrical member, such as a valve stem, that can be used for determining the axial loading on the substantially cylindrical member.
In many industries it is important to measure the variable dynamic axial loads that may be imposed on a cylindrical member or shaft. This is especially true in the nuclear power industry where motor operated valves are used extensively and monitoring of the various operating parameters of the valves are required by regulatory agencies. Motor operated valves are comprised generally of an electric motor driven valve actuator that is connected to a valve stem and a valve yoke that partially surrounds the valve stem. Rotation of a nut attached to the valve stem by the valve actuator will move a valve plug into a closed, open or intermediate position with respect to a valve seat in the body of the valve.
It has been found that one of the best ways to monitor the dynamic forces and events that occur during the operation of a motor operated valve is by direct measurement of the valve stem axial loads using either axial or diametral strain gages.
It is well known that one can calculate the axial load or strain in a valve stem or any other similar substantially cylindrical member, by measuring changes in the diameter of the valve stem or cylindrical member. The ratio of the diametral change to axial elongation for a material, referred to as Poisson's ratio, is known. Therefore, by measuring the diametral change on the valve stem or cylindrical member, axial strain and valve stem axial load can be easily determined.
U.S. Pat. Nos. 4,911,004; 4,930,228; 4,936,150 and 5,123,283 describe several prior devices and systems that have been developed for measuring axial strains in a valve stem or similar cylindrical member.
Some of these prior devices were a clamp type of strain gage that used two clamp assemblies to attempt to define a reference gage length axially along the valve stem. Such devices were usually expensive, comprised of many parts and often difficult to mount on the exposed portion of a valve stem.
Many of the other prior devices designed as clamps to fit around an exposed portion of the valve stem or cylindrical member to measure diametral strains were usually designed so that any diametral strains in the valve stem would cause the clamp to deflect. This clamp deflection was then measured and calibrated to diametric strains. The measurement of clamp deflection however, requires the device to be flexible enough to provide a reasonable level of output signals. However, the flexible nature of these devices often led to undesirable and erroneous output signals due to the torsional loads that are also applied to the valve stem during operation of the valve. Many of these prior devices were not able to distinguish strains induced in the valve stem due to torsional loads from those induced in the valve stem through axial loads and thrust.